The present invention relates to a nuclear magnetic resonance spectrometer and, more particularly, to a nuclear magnetic resonance spectrometer capable of eliminating deleterious effects, such as occurrence of sidebands upon spinning of the sample tube, which result from the eccentricity of the sample tube, asymmetry of the shape of the tube, and other causes.
The resolution of an NMR spectrum depends on the homogeneity of the static, or dc, magnetic field which is applied to the sample in the direction of Z-axis. Therefore, in an ordinary nuclear magnetic resonance spectrometer, the sample tube is rotated about the Z-axis in the static magnetic field to equalize and neutralize the effects resulting from the inhomogeneity of the static field. This technology is known as "spinning." In the following situations, however, peaks, termed spinning sidebands, occur at the positions shifted from the peaks of an NMR spectrum by integral multiples of the rotational frequency of the sample, because the obtained NMR signal is subjected to modulation, and hence a difficulty is introduced in the analysis of the spectrum. (a) Excessively large inhomogeneous components of a static magnetic field which are not symmetrical with respect to the axis of rotation of the sample are left uncancelled. (b) The RF magnetic field (pulses) which is produced during observation is not spacially homogeneous in intensity. (c) The axis of rotation of the sample tube does not coincide with the central axis of the sample because of insufficient accuracy with which the sample tube is fabricated or for other causes. The spinning sidebands attributable to the phenomena (a) and (b) above can be almost completely eliminated by adjustment while the instrument is in use. However, the sidebands arising from the situation (c) above cannot be eliminated by electromagnetic adjustment. Accordingly, the sole solution to this problem is to fabricate the spectrometer with higher accuracy or use a sample tube that has been produced with higher accuracy.
In general, the inhomogeneous components of a static magnetic field applied to the sample, i.e., the gradient component Bz of the field, can be given using Legendre polynominals as follows: EQU Bz=A.sub.1.sup.0 +A.sub.2.sup.0 z+A.sub.3.sup.0 (z.sup.2 -1/2(x.sup.2 +y.sup.2)) EQU +A.sub.4.sup.0 z(z.sup.2 -(z/3)(x.sup.2 +y.sup.2))+A.sub.2.sup.1 x+B.sub.2.sup.1 y+A.sub.3.sup.1 zx EQU +B.sub.3.sup.1 zy+A.sub.3.sup.2 (x.sup.2 -y.sup.2)+B.sub.3.sup.2 xy+ . . .
where A.sub.1.sup.0, B.sub.2.sup.1, and so on are the coefficients of the terms.
Therefore, in an ordinary nuclear magnetic resonance spectrometer, a number of compensating coils, known as shims, which are energized with electric current are placed in the magnetic field to produce compensating gradient magentic fields which serve to cancel out all the terms of the above formula. Rotating the sample about the Z-axis as described above is tantamount to averaging out the horizontal components, i.e., A.sub.2.sup.1, B.sub.2.sup.1, A.sub.3.sup.1, etc., which are not rotationally symmetrical. Unfortunately, rotation of the (solid) sample presents the disadvantage that the shims acting on the horizontal gradient components of the field are made ineffective. In this way, it is impossible to enhance the resolution of spectrum simply by adjusting the X shims and Y shims constituting the shims which act to correct the horizontal gradient components of the field. For example, in the case where rotation of the sample induces apparent horizontal gradient components of the magnetic field when viewed from the sample on account of the inhomogeneity of the sample itself resulting from deposition or other phenomenon or because of the aforementioned situation (c), the effects of the horizontal gradient components of the field cannot be removed by the conventional electromagnetic shim system.